In modern community antenna television (CATV) distribution networks (also known as broadband networks), there have been recent shifts to multi-redundant centralized powering and to the extension of networks by using fiber optic transmission cables. In order to provide these expanding networks and power hungry equipment with AC power from a single source, the AC voltage in these systems is being increased from the 60 volts to 90 volts. The resulting networks tend to have higher voltage drops along the distribution tree than previous networks, and have been plagued by unstable conditions wherein DC power supplies begin drawing increased current and overloading the AC power delivery capabilities of the network.
In an ideal system consisting of a power source and a load, maximum power transfer occurs when the impedance of the load matches the impedance of the power supply. If the load resistance increases then current through the system will decrease and less power will be transferred. If the load resistance decreases then the voltage across the load will decrease and less power will be transferred.
In a CATV system, long conductor distances have significant series resistance so that remote portions of these systems tend to have voltages far below the voltage at the AC power source. In addition AC voltage in the system may vary dynamically due to momentary power interruptions, sags, system maintenance, adding new users, activation of various equipment in the system, temporary overloads, or "sheath currents" caused by imbalances in the power distribution system.
In CATV, at nodes, signal amplifiers, and customer interface units (CIUs), AC power is converted to DC power by a power supply, and the DC power is used to operate various electronic equipment. Switched-mode power supplies (SMPSs) are used in these applications because they are able to maintain high conversion efficiency over a wide range of input voltage. Unfortunately, this constant power characteristic provokes instability in the AC power distribution network because the input impedance of the DC power supplies is dynamically negative. That is, a decrease in the supply voltage causes an increase in the current demanded by the power supplies which causes increased voltage drop through the distribution system and results in a further decrease in the voltage at the input of the DC power supply.
In modern DC power supplies, as the AC input voltage drops towards a minimum level, the current demanded by the power supply increases, and then the DC output becomes unregulated when the AC input voltage dips below the DC power supply's minimum level. At that time, the current demanded by the DC power supply stops increasing and the network enters a quasi-stable state in the overloaded condition. The increased current further reduces the input voltage throughout the branch of the network. Thus, as a DC power supply in one branch of the power distribution network becomes unregulated, then other DC power supplies in the branch tend to also become unregulated resulting in cascade failures. A large number of CATV cable customers are subjected to service degradation or interruption. In addition, the recovery voltage at which the network regains its stability is significantly higher than the voltage at which it originally became unstable, so that it is difficult to regain stability. Currently, the typical response to instability is to shut down the entire system or at least a portion of the system that can be remotely shut down, and then power distribution is reinitiated in the shut down portion.
Those skilled in the art are directed to the following citations. "Powering Stability in 90 Volt Networks" by Peter Deierlein in 1996 NCTA Technical Papers describes the power distribution system instability problem and suggests that "a latching shutdown function (with delayed restart) is added to the power supplies". European Patent Application 0 582814 A2, to Newton suggests "A regulator circuit that allows recovery in a controlled manner in the shortest possible time from occasional power line disturbances".
Those skilled in the art are referred to U.S. Pat. No. 4,937,722 to Deierlein.
The above citations are hereby incorporated herein in whole by reference.